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Endo-siRNAs restrain proteasomes to sustain translation in oocytes

Ying Liu

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Vita > Cutting Edge > DOI: 10.15302/vita.2026.03.0018
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Endo-siRNAs restrain proteasomes to sustain translation in oocytes

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In a study published in Vita, Xue et al. show that endogenous siRNAs guide Argonaute proteins to suppress proteasome subunit expression in oocytes of C. elegans and mice. This regulation limits 26S proteasome assembly, preserves ribosomes, and enables the translation of transcription factors required for zygotic genome activation, revealing a conserved siRNA-proteasome-ribosome axis underlying the maternal-to-zygotic transition.

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The maternal-to-zygotic transition is a pivotal developmental event during which control of embryogenesis shifts from maternally deposited RNAs and proteins to transcription from the zygotic genome. Successful zygotic genome activation (ZGA), therefore, requires that oocytes be equipped with sufficient translational machinery — ribosomes and translation factors — to produce the transcription factors that initiate the embryonic program. How oocytes protect this translational capacity has remained unclear. Xue et al.1 now show that endogenous small interfering RNAs (endo-siRNAs) restrain proteasome activity in oocytes, preventing premature degradation of ribosomal proteins and preserving the translational competence required for embryogenesis.
Endogenous siRNAs are abundant in animal oocytes, yet their physiological roles have remained largely unclear beyond transposon silencing and genome defense2,3. In C. elegans, the Argonaute CSR-1 associates with 22G-siRNAs and is essential for fertility. In mice, AGO2 loaded with DICERᴼ-generated endo-siRNAs is likewise required for oocyte competence4,5. Loss of either Argonaute results in complete embryonic lethality, but the underlying mechanism has remained unresolved. Although spindle assembly defects were previously proposed to contribute to this phenotype, such abnormalities occur in only a subset of embryos, and restoring spindle assembly fails to rescue viability6. These observations suggested that endo-siRNAs play a more fundamental physiological role in oocytes.
Mechanistic insight emerged when the authors identified proteasome subunit transcripts as prominent targets of Argonaute–siRNA complexes (Fig. 1). In C. elegans, CSR-1-associated siRNAs preferentially target mRNAs encoding components of the proteasome, particularly the 19S regulatory subunit. Loss of CSR-1 increases the abundance of these transcripts and proteins, leading to enhanced 26S proteasome assembly and activity specifically in oocytes. A similar regulatory logic operates in mouse oocytes. AGO2-associated endo-siRNAs cleave numerous coding transcripts, including multiple proteasome subunit mRNAs. Notably, cleavage can occur despite imperfect complementarity, suggesting that mammalian endo-siRNAs may recognize a broader range of targets than previously appreciated.
Elevated proteasome activity has profound consequences for oocyte physiology. In both worms and mice, Argonaute-deficient oocytes show accelerated degradation of ubiquitinated ribosomal proteins and translation factors. Proteomic analyses indicate that these proteins are bona fide proteasome substrates whose mRNA levels remain unchanged, demonstrating that their reduced abundance results from increased proteolysis. Consistently, ribosome profiling reveals a global decrease in ribosome occupancy in mutant germ cells, with transcription factors among the most affected transcripts. In C. elegans, translation of nuclear hormone receptors and homeobox factors is particularly reduced, whereas in mice several ZGA regulators — including YY1, SP1 and GABPB1 — are similarly affected. Electron microscopy further confirms a reduction in ribosome abundance in AGO2-deficient oocytes. Functional experiments reinforce this model: combined knockdown of key transcription factors phenocopies the developmental arrest observed in Argonaute mutants.
A particularly compelling observation is that embryonic lethality can be partially rescued by limiting proteasome activity. In worms, treatment with the proteasome inhibitor MG132 or genetic reduction of a 19S proteasome subunit restores hatching in a substantial fraction of embryos. In mice, low-dose MG132 treatment of AGO2-deficient oocytes followed by intracytoplasmic sperm injection allows many embryos to progress through early developmental stages. Rescue is maximal when proteasome activity is restored to nearly wild-type levels, whereas excessive inhibition becomes detrimental, highlighting the importance of balanced proteostasis during early development7. Additional experiments further support this model: restoring translational capacity rescues embryonic lethality and reactivates ZGA gene expression.
This study reshapes our understanding of endo-siRNAs in several ways. First, it identifies a physiological role of mammalian oocyte endo-siRNAs beyond transposon control. Proteasome regulation and transposon silencing appear to represent parallel downstream pathways, as proteasome inhibition does not restore transposon expression in AGO2-deficient oocytes. Second, the work provides a mechanistic explanation for why loss of CSR-1 or AGO2 causes complete embryonic lethality despite producing spindle defects in only a subset of embryos. Third, it reveals that retrotransposon-derived siRNAs in mouse oocytes can cleave coding transcripts despite imperfect complementarity, highlighting a regulatory role of transposon-derived small RNAs in proteostasis, echoing McClintock’s concept of “controlling elements”8,9.
The broader implications are considerable. The siRNA-proteasome-ribosome axis described here highlights a previously unrecognized mechanism for maternal quality control: oocytes use small RNA pathways not only for genome defense but also to modulate protein degradation machinery, thereby preserving the translational capacity required to initiate embryogenesis. Conservation of this mechanism from nematodes to mammals suggests that similar regulatory principles may operate broadly in animal reproduction. Dysregulation of this pathway could therefore contribute to certain forms of oocyte incompetence or early embryonic failure in humans10,11. Several important questions remain. Does the balance between proteasome activity and ribosome preservation shift dynamically during different stages of oogenesis? How are proteasome subunit transcripts selectively targeted by endo-siRNAs, and what determines the specificity of Argonaute-mediated cleavage in oocytes? Addressing whether a comparable mechanism operates in human oocytes and whether modulation of proteasome activity could improve outcomes in assisted reproduction will be important directions for future study.

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The Author(s) 2026. Published by Higher Education Press. This is an Open Access article distributed under the terms of the CC BY license (https://creativecommons.org/licenses/by/4.0/).

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Liu, Y. Endo-siRNAs restrain proteasomes to sustain translation in oocytes Vita https://doi.org/10.15302/vita.2026.03.0018 ()
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